1. Field of the Invention
The present invention relates to molding machines, and more particularly to injection molding machines.
2. The Prior Art
In 1992 the world production of plastics reached 102 million m.sup.3 /year (total value of over $300 billion), while production of steel was 50 million m.sup.3 /year (total value of about $125 billion). Furthermore, from 1980 to 1990 plastics production increased by 62% while that of steel decreased by 21%. It is clear that polymers are the fastest growing structural materials. Polymer alloys and blends constitute more than 30% of the commercial polymer market, and with the constant annual growth of some 9% (four-fold of the growth rate of the plastics industry as a whole), their importance is destined to increase.
The number of polymer blend patents issued each year has continued to increase rapidly. In 1993 it reached 3000 (at least 50% from 26 major polymer manufacturers in Japan), representing the outcome of multi-billion dollar investments in industrial research. Obviously, to justify these expenditures, the blends must provide appropriate returns. Indeed, some of these materials achieved spectacular financial success. It was reported that by 1982 the annular global sales of PPE/PS blends exceeded one billion dollars.
It has been recognized that blending offers several economic benefits. For example, it makes it possible to generate, rapidly and economically, a desired set of properties: mechanical, chemical, barrier to permeation by gases or liquids, etc., fulfilling customer requirements. It also offers better processability of difficult-to-form, high performance polymers, by reducing the viscosity and/or the processing temperature. The achieved improvement in processability leads to better product uniformity, and therefore to reduction of scrap. The inherent recyclability of thermoplastic polymer blends achieved by regeneration of morphology, as well as enhanced plant flexibility and productivity, translate into profitability.
With the growing importance of reactive processing, the blending technology makes it possible to offer new types of materials, characterized by controlled chemical constitution and morphology, which can be precisely tailored to specific requirements. There is a growing need for cost-competitive materials where the total cost-to-performance ratio encompasses all aspects: material, compounding, forming, assembling, and recycling costs. As the history of polymer blends indicates, the blending technology is particularly well suited to accomplish this task.
Analysis of more recent patents indicates that processability is becoming the most important property. This is understandable since the emphasis continuously shifts to high performance, specialty resins requiring high processing temperatures and pressures. Since frequently the processing temperature is near or even above the decomposition temperature, the blending constitutes the only sensible solution. It is expected that in the future, electrically, magnetically, and optically active polymeric blends will also become important.
From time to time in applied science there is a condition where technology greatly lags practice. The injection molding field is at such a nexus. The nature of the industry has drastically changed, but the content of the published research has not. Over the past 7-9 years in the plastics field, product performance and properties have been obtained by blending, alloying and reactive compounding, rather than creating new polymers. One reason is that any new product requires an entire vertical chain of suppliers and customers in order to bring the product to market. This process is expensive and time consuming. Furthermore, the new polymers may require more complex architecture in order to satisfy the performance requirements. There is also an inverse relationship between polymer complexity and recyclability.
Turning now to the molding field, a vast majority of the current literature in the field of injection molding is oriented towards the mold end of the operation. A review of the past 7 years of the Society of Plastics Engineers Annual Technical Conference proceedings reveals that a full third of the articles focus on injection molding, but that over 95% of those articles deal with the mold processes alone. The few papers that address the screw part of the machine often are focused on control algorithms and consider the screw part as a black box. This was sufficient when injection molders were primarily processing homopolymers or special copolymers, where the screw part of the machine did very little to alter the morphology. This is no longer true. Dow Chemical in Europe is developing the expertise to have its customers self-color (mix master batch with virgin resin) in the injection molding machine. This will reduce customer's costs, reduce recycle problems, and improve product properties by the elimination of one heat cycle (the extrusion step). Black & Decker experienced a difficult development period in the mixing uniformity of the yellow color for the DeWalt line of products. Various screw manufacturers, like Spirex, have attempted to address the mixing issues in the injection molding machine. Most of the work has been heuristic, lacking a fundamental scientific basis. Furthermore, much of the current research in blending of polymers seeks to develop stable morphologies through compatibilization or reactive processing, so that the properties survive through the injection molding cycle.
Since the current trend of the industry is to satisfy customer product requirements through blending and reactive compatibilization, the demand for mixing performance during the process has expanded. Currently, the material path for production of the engineering plastics in the polymer industry encompasses three steps: 1) Basic Resin Synthesis-Produces the base versions of a polymer (homopolymer), whether a commodity or an engineering polymer. The process is characterized, often by the large-scale mass production of the polymer and is usually done by major petrochemical companies like Exxon, Shell, Monsanto, etc. 2) Extrusion-Production of specific polymer with particular morphology and properties for final process (addition of fillers, colorants, and other minor components for specific applications). PVC, for example, requires up to 15 different additives to be blended into the basic polymer to produce the desired properties. Since this step requires extensive mixing and compounding, a machine like a co-rotating twin screw extruder would be used. An extrusion line with a 57 mm machine, with a capacity of 300 lb./hr., would cost over $500,000. 3) Injection Molding--The pellets are purchased from the compounder and then processed into the final part. Most of the technical emergencies a scientist working for a resin producer solves is for an injection molder, when their final product is not to specification.
Schematically the material flow in the polymer industry is shown in FIG. 1. FIG. 1 shows the inherent inefficiencies of the industry. Each step adds to the base material at the cost of energy consumption, heavy machinery investment and any resultant off-specification material, which constitutes the recycle stream. Also, many processes in the extrusion and injection molding steps are duplicates of each other. Each machine must melt, pressurize and pump the material. The existing injection molding machines have a single screw; these machines are inherently poor mixers. Blending and compounding have to be performed in a twin screws machine which has superior mixing characteristics. By having to go through the extrusion step, though, the material goes through another heat cycle, which degrades the material and another recycle stream comes in to existence.